🔋 Voltage Drop Calculator
Voltage drop for copper or aluminum wire by AWG or kcmil, current and length — single phase, three phase or DC — plus the smallest wire within 3%.
Length is the distance from the panel or source to the load, one way — the formula accounts for the return path.
Over 5% — consider a larger wire, a shorter run or a higher voltage.
VD = 2 × 12.9 × 16 A × 100 ft ÷ 6,530 cmil. Voltage drop is only one check: the wire must also be rated for the current (ampacity), the insulation temperature and the breaker — confirm with the electrical code and a licensed electrician.
What Voltage Drop Calculator Does
This voltage drop calculator shows how many volts are lost in a run of wire and what percentage of the supply that is, for copper or aluminum conductors from 18 AWG up to 750 kcmil. It handles single-phase AC, three-phase AC and two-wire DC circuits — useful for everything from a garage subpanel to a 12-volt solar or RV system.
Beyond the drop itself it answers the two questions that come next: what is the smallest wire that keeps the drop within your target (3% by default), and how long can the run be with the wire you have in mind. The formula is shown with your numbers so the result can be checked.
How to Use Voltage Drop Calculator
- Choose single phase, three phase or DC
- Pick copper or aluminum and the wire size
- Enter the source voltage, load current and one-way length
- Read the voltage drop in volts and percent
- Check the smallest wire that stays within your target, and the longest run for your wire
Formula Used by Voltage Drop Calculator
Circular-mil method
VD = 2 × K × I × L ÷ CM (single phase, DC) · VD = √3 × K × I × L ÷ CM (three phase)
- K
- Resistivity: about 12.9 Ω·cmil/ft for copper, 21.2 for aluminum
- I
- Load current in amps
- L
- One-way length in feet
- CM
- Conductor area in circular mils
Worked example
16 A at 120 V over 100 ft of 12 AWG copper (6,530 cmil).
- 2 × 12.9 × 16 × 100 = 41,280
- 41,280 ÷ 6,530 = 6.32 V
- 6.32 ÷ 120 = 5.3%
Result: Too much for a branch circuit; 8 AWG brings it to about 2.1%.
Common Wire Sizes
Solid-conductor equivalents from the AWG definition; stranded conductors have the same circular-mil area.
| Size | Circular mils | Diameter (bare) |
|---|---|---|
| 14 AWG | 4,110 | 0.064 in |
| 12 AWG | 6,530 | 0.081 in |
| 10 AWG | 10,380 | 0.102 in |
| 8 AWG | 16,510 | 0.128 in |
| 6 AWG | 26,240 | 0.162 in |
| 4 AWG | 41,740 | 0.204 in |
| 2 AWG | 66,360 | 0.258 in |
| 1/0 AWG | 105,600 | 0.325 in |
| 4/0 AWG | 211,600 | 0.460 in |
How to Read Your Result
Why voltage drop matters
Equipment is designed for its rated voltage. Motors on a long, undersized run draw more current and run hotter, heaters and lights lose output, and electronics can reset. The lost voltage is also wasted as heat in the wire, which costs money on every hour of use.
Low-voltage systems
At 12 V the same current and distance cause the same volts of drop as at 120 V — but that is ten times the percentage. That is why 12-volt solar, RV and boat wiring needs much larger wire than household circuits for the same load, and why raising the system voltage is often the cheaper fix.
Limitations & Accuracy Notes
- Uses DC resistance at typical operating temperature; for large AC conductors, reactance adds a little more drop.
- Does not determine ampacity, breaker size or code compliance — size wire for current first.
- Consult the electrical code and a licensed electrician for installations.